This invention relates to lamps with a protective envelope coating and, more particularly, to an improved method for applying such a coating on the glass envelope of a lamp. The method of the invention is particularly useful for applying a UV curable photopolymer as a protective coating on the exterior surface of a photoflash lamp.
A typical photoflash lamp comprises an hermetically sealed glass envelope, a quantity of combustible material located in the envelope, such as shredded zirconium or hafnium foil, and a combustion supporting gas, such as oxygen, at a pressure well above one atmosphere. The lamp also includes an electrically or percussively activated primer for igniting the combustible material to flash the lamp. During lamp flashing, the glass envelope is subject to severe thermal shock due to hot globules of metal oxide impinging on the walls of the lamp. As a result cracks and crazes occur in the glass and, at higher internal pressures, containment becomes impossible. In order to reinforce the glass envelope and improve its containment capability, it has been common practice to apply a protective lacquer coating on the lamp envelope by means of a dip process. To build up the desired coating thickness, the glass envelope is generally dipped a number of times into resin, typically cellulose acetate. After each dip, the lamp is dried to evaporate the solvent and leave the desired coating of cellulose acetate, or whatever other plastic resin is employed.
In the typical solvent dipping process for applying protective coatings, a large number of photoflash lamps are loaded on a rack and then successively dipped in a solvent solution and oven dried three or four times to build up the desired coating thickness. Such a process is time consuming, uses a large area of production floor space, and involves considerable hand labor, all of which adds significantly to manufacturing costs. Further, as the lacquer solution includes a highly flamable solvent, such as acetone, an inadvertent flashing of one of the lamps in either the dip bath or drying oven can ignite the solvent fumes. To substantially reduce or eliminate this hazard, costly automatic extinguishing equipment must be employed. In the event of a solvent ignition, the resulting down time and consumption of fire extinguishing chemicals also adds to the manufacturing costs.
Another approach to providing a more economical and improved containing vessel is described in U.S. Pat. No. 3,893,797, wherein a thermoplastic coating, such as polycarbonate, is vacuum formed onto the exterior surface of the glass envelope. The method of applying the coating comprises: placing the glass envelope within a preformed sleeve of the thermoplastic material; drawing a vacuum in the space between the thermoplastic sleeve and the glass envelope; and simultaneously heating the assembly incrementally along its length, whereby the temperature and vacuum cause the thermoplastic to be incrementally formed onto the glass envelope with the interface substantially free of voids, inclusions and the like. Although this method provides an optically clear protective coating by means of a significantly faster and safer manufacturing process which may be easily integrated on automated production machinery, it does present the disadvantage of requiring preformed plastic sleeves which must be individually designed for each different lamp type, made or purchased, stocked, and fed into the production apparatus which applies the sleeves onto the envelopes.
Further approaches toward providing improved protective coatings for lamps are described in the aforementioned Dow et al patent applications, which relate to coatings including UV curable photopolymers. For example, in the Dow et al application Ser. No. 753,255, a method of coating a flashlamp with a photopolymer is described comprising the following steps. First, the lamp is held vertically with the base up and dipped into a vat of the photopolymer at 60.degree. C. and extracted very slowly, the dip process taking about 45 seconds. The resulting coating thickness is about 0.020 inch. According to an alternative method described in the same Dow et al application, the flashlamp, while revolving, is sprayed with the liquid photopolymer and then transferred directly into the ultraviolet lamp chamber. Dow et al application Ser. No. 699,139 is somewhat similar except that either long or short strands of fiber glass are employed to reinforce the photopolymer coating.
An immersion process for applying a UV cured coating on a photoflash lamp is also described in a published Japanese patent application identified as Public Disclosure Number 52-7720 and having a publication date of Jan. 21, 1977; the corresponding U.S. application Ser. No. 592,194, filed June 17, 1976, was abandoned, and a continuation of a continuation-in-part thereof issued as U.S. Pat. No. 4,076,489 on Feb. 28, 1978.
A somewhat critical aspect of the aformentioned UV cured coatings is that the shape and uniformity of thickness depends on the flow characteristics of the photopolymer resin as influenced by the force of gravity, orientation of the lamp after coating, and viscosity of the resin. Changes in resin viscosity resulting from changes in temperature affect both the repeatability of the shape of the coating and the uniformity of thickness. These irregularities are retained once the coating is hardened. In the case of UV cured coatings used to protect flashlamps from rupture at the time of flashing, thin coating regions resulting from improper resin distribution can result in containment failures. The comparative integrity or containment for various types of vessel constructions can be evaluated by the use of special test lamps, such as described in U.S. Pat. No. 3,955,912 assigned to the present assignee, which controllably induce bursting of the lamp upon ignition. Accordingly, such test lamps were coated with UV cured photopolymers by means of various immersion techniques and compared in flash tests with similar test lamps coated in accordance with the present invention. The results showed that the repeatable uniformity of the coating applied in accordance with the invention provided significantly superior containment characteristics over the lamps coated with the prior art immersion techniques.